Switching voltage regulators are widely used in modern electronic systems for a variety of applications such as computing (server and mobile) and POLs (Point-of-Load Systems) for telecommunications because of their high efficiency and small amount of area/volume consumed by such converters. Widely accepted switching voltage regulator topologies include buck, boost, buck-boost, forward, flyback, half-bridge, full-bridge, and SEPIC topologies. Multiphase buck converters are particularly well suited for providing high current at low voltages needed by high-performance integrated circuits such as microprocessors, graphics processors, and network processors. Buck converters are implemented with active components such as a pulse width modulation (PWM) controller IC (integrated circuit), driver circuitry, one or more phases including power MOSFETs (metal-oxide-semiconductor field-effect transistors), and passive components such as inductors, transformers or coupled inductors, capacitors, and resistors. Multiple phases (power stages) can be connected in parallel to the load through respective inductors to meet high output current requirements.
Accurate knowledge of the current being supplied by a switching voltage regulator is required for safe, robust operation. Incorrect current measurement can lead to premature tripping over-current protection (OCP), which is undesirable in high-reliability applications such as servers in data centers, or failure to trip OCP which can lead to catastrophic device failure. In either case, incorrect current information causes an incorrect output voltage set point that may be out of compliance with product specifications.
The standard technique for ascertaining the current through an inductor in a switching voltage regulator is known as DCR sensing which exploits the non-ideal DC resistance (DCR) of the inductor. The DCR of an inductor is specified at nominal conditions in the datasheet for off-the-shelf components, or may be calculated or measured in custom designs. With DCR sensing, a resistor-capacitor (RC) sense network is connected in parallel with the inductor and the voltage across the capacitor represents the voltage across the DCR, meaning the current through the inductor is related to the ratio of the capacitor voltage to DCR value. For a static, DC current through the load, the DC value across the capacitor is proportional to the DC current through the inductor and the DCR of the inductor. For dynamic currents, such as a changing current through the load or a ripple current through the inductor, the instantaneous value across the capacitor is proportional to the instantaneous current through the inductor if the time constant of the RC network matches that of the inductance and DCR of the inductor. However, inductance and DCR vary with temperature, meaning the current information is only accurate at the temperature where the time constants match.
Many conventional switching voltage regulators use a negative temperature coefficient thermistor (NTC) network to augment the DCR measurement. The NTC network for DCR compensation typically is separate from the NTC network for over temperature protection (OTP), increasing overall system cost, component count and signal routing complexity. In addition, the temperature compensation NTC network does not contain useful information. Instead, the temperature compensation NTC network is a manually tuned unintelligent network. As a result, the network is specific to the platform on which it is used. Changing design parameters with thermal implications (e.g. board layer count, air flow, components with lower efficiency, etc.) require a hardware change to re-tune the NTC network. Also, component tolerances of the NTC network introduce additional error in the current measurement and the NTC technique for DCR compensation is only viable for DC measurements. As such, the dynamic or AC value of current is uncompensated.